1. Field of the Invention
The present invention relates to a coil device such as a transformer and an inductor to be used for electronic equipment, switching power supply apparatus or the like, and the switching power supply apparatus using it.
2. Description of the Related Art
Switching power supply apparatus have been widely used for a power supply units of various electronic devices such as personal computers, facsimile machines or the like. Especially, switching power supply apparatus including an isolated forward converter type as a DCxe2x80x94DC converter are suitably employed in laptop computers or the like.
Such a switching power supply apparatus including an isolated forward converter generally comprises a power transformer, a driving transformer and a choke coil. These coil elements use coils and core parts, which are bulky and require substantial spaces to be mounted on a circuit board. As a result, it is very difficult to miniaturize the circuit board and switching power supply apparatus itself.
In addition, transformers and choke coils are relatively expensive elements due to the fact that manufacturing methods of these elements include a step which cannot be automatically processed. It is difficult to reduce the cost of the switching power supply apparatus, accordingly.
The present invention can solve the aforementioned drawbacks associated with the conventional coil elements and provides a coil device capable of saving space, miniaturizing size, and reducing cost. The present invention also provides a switching power supply apparatus which uses the coil device and can be produced at a low cost.
The coil device comprises a core, first printed coil and a second printed coil. The core has an intermediate leg and at least a pair of external legs arranged with equal intervals from the intermediate leg. The first printed coil is provided on a first substrate which has an intermediate hole and a pair of external holes respectively inserted in the intermediate leg and the pair of external legs of the core, the first printed coil being wound around the intermediate hole. The second printed coil is provided on a second substrate which has an intermediate hole and a pair of external holes respectively inserted in the intermediate leg and the pair of external legs of the core, the second printed coil including a pair of external printed coils which are connected in series and are respectively wound around the pair of the external holes by the same number of turns but in the opposite directions.
According to this structure, the pair of external printed coils of the second printed coil generate magnetic fields in the opposite direction to each other in the intermediate leg such that the magnetic fields are canceled with each other and induce no induction voltage in the first printed coil, and induction voltages induced in the external printed coils of the second printed coil by the magnetic flux generated by the first printed coil is canceled to be equivalently 0V, whereby the first printed coil and the second printed coil form separate inductors which are not magnetically coupled.
In this invention, the number of total legs of the core includes three for a pair of external legs to one intermediate legs, four in the case of a pair of external legs and one external leg, or five in the case of two pairs of external legs.
First, a case where the number of total legs of the above-described core is three, is described. The case where the number of total legs is three is of the structure where a pair of external legs are arranged with equal intervals with the intermediate leg as the base point. Thus, a conventional E-shaped core in which the external legs are arranged in the direction opposite to each other by 180xc2x0 with the intermediate leg as the center is included in this case of three legs.
Among a pair of external legs, one external printed coil wound around the one external leg and the other external printed coil wound around the other external leg are wound in the same direction and inversely connected in series, or wound in the reverse direction and forwardly connected in series to form one second printed coil (a pair of external printed coils).
Thus, when current flows in the first printed coil (intermediate printed coil) wound around the intermediate leg, magnetic fluxes parallel in the same direction passing through the one external leg and the other external leg are generated. The voltages induced in the one external printed coil and the other printed coil to be interlinked with the magnetic fluxes are in the opposite direction to each other, and canceled, and no equivalently induced voltages are outputted in both terminals of the second printed coil (a pair of external printed coils).
When current flows in the second printed coil (a pair of external printed coils), the magnetic fluxes which are generated from the one external printed coil and the other external printed coil and pass through the intermediate leg are in the opposite direction to each other, and cancel each other, and no induced voltage is outputted in both terminals of the first printed coil (intermediate printed coil).
Thus, the first printed coil (intermediate printed coil) wound around the intermediate leg and the second printed coil (a pair of external printed coils) wound around a pair of external legs constitute coil devices which are individual parts wound by cores as if they are independent from each other where the core is commonly used, and the interlinked magnetic fluxes or the induced voltages cancel with each other.
Second, a case where the number of total legs of the above-described core is four is described. In the case of four legs, which is a case where one leg (the fourth leg) is added to the above-described case with three legs, and the fourth leg is arranged on a line to perpendicularly divide into two the line connecting the external legs of a pair of external legs to each other. Thus, the fourth leg is in a relationship similar to that of the above-described intermediate leg in the form relative to a pair of external legs, and at the same time, similar to that of the above-described intermediate leg, in effect, so to speak, the second intermediate leg. Further, the fourth leg forms the relationship of a U-shaped core in relation to the intermediate leg, and the printed coils wound therearound forms the relationship between the primary coil and the secondary coil. The external printed coil wound around the fourth leg is paired with the intermediate printed coil wound around the intermediate leg, that is, a similar relationship to that of a pair of above-described external printed coils can be formed by connecting the printed coils in series so that the directions of the magnetic fluxes passing through the legs become opposite to each other. Two pairs of external printed coils can be formed thereby. In addition, two transformers can be formed by forming two pairs of external printed coils equivalent or similar to each other.
Third, a case where the number of total legs of the above-described core is five is described. In the case of five legs, which is a case where two sets of a pair of external legs, i.e., two pairs of external legs are provided, and the effect of two pairs of external printed coils (two second printed coils) wound around two sets of the external legs to the intermediate leg is the same as the relationship between the above-described intermediate leg and a pair of the external printed coils (one second printed coil). Further, the relationship between two sets of the external printed coils, i.e., between a pair of one external printed coils and a pair of the other printed coils is that the mutually induced voltages equivalently cancel each other.
Further, a case where the number of total legs is six or more is possible, and the description for the above-described cases of three to five legs is analogously applicable.
According to another embodiment of the invention, the coil device is further provided with a third printed coil equal to or similar to the above-described first printed coil, wherein the above-described first printed coil is magnetically coupled with the above-described third printed coil to form the first transformer.
In this embodiment, the third printed coil (intermediate printed coil) wound around the intermediate leg around which the first printed coil is already wound is magnetically coupled with the above-described first printed coil to constitute the first transformer (intermediate transformer) in the relationship between the primary coil or the secondary coil of the transformer. The magnetic flux generated from the third printed coil of the first transformer is induced in the second printed coil to generate the voltage, but the voltage is canceled to be 0V. The magnetic flux generated from the second coil and passing through the intermediate leg is canceled, and no voltage due to the induction is generated in the third printed coil of the first transformer.
According to still another embodiment, the coil device is further provided with a fourth printed coil equal to or similar to the above-described second printed coil, wherein the above-described second printed coil is magnetically coupled with the above-described fourth printed coil to form the second transformer.
In this embodiment, the fourth printed coil (a pair of external printed coils) further wound around a pair of external legs around which the second printed coil is already wound is magnetically coupled with the above-described second printed coil to constitute the second transformer (external transformer). The magnetic flux generated from each printed coil of the fourth printed coil of the second transformer and passing through the intermediate leg is canceled, and no voltage due to the induction is generated in the first printed coil (intermediate printed coil). The magnetic flux generated from the first printed coil is interlinked with the fourth printed coil of the second transformer to generate the voltage, but this voltage is canceled to be 0V.
According to still another embodiment, a coil device is provided with a third printed coil equal to or similar to the above-described first printed coil wherein the above-described first printed coil is magnetically coupled with the above-described third printed coil to form a first transformer, and is further provided with a fourth printed coil equal to or similar to the above-described second printed coil wherein the above-described second printed coil is magnetically coupled with the above-described fourth printed coil to form the second transformer.
In this embodiment, the first transformer (intermediate transformer) having two intermediate printed coils around the intermediate leg of the core is formed, and the second transformer (external transformer) having two pairs of the external printed coils around a pair of external legs is formed. A pair of the intermediate printed coils of the first transformer and a pair of the external printed coils of the second transformer are wound around one core, but similar to the effect of the invention already described, the interlinked magnetic fluxes cancel each other, or the voltages due to the induction cancel each other to form two independent transformers jointly using one core.
According to still another embodiment, a switching power supply apparatus includes a transformer for power transmission having a primary coil and a secondary coil, a main switch to control the current flowing from the input power supply into the primary coil, a rectifying and smoothing circuit which includes a choke coil and rectifies and smoothes the output of the secondary coil of the transformer for power transmission, and a control circuit to control the action of the main switch, the transformer for power transmission and the choke coil being formed of the coil device described above.
In this embodiment, the transformer for power transmission and the choke coil for rectifying and smoothing operation sharing the core comprise the coil device provided with the transformer which is formed on one core and equivalently generates no output voltage due to the induction. A core fitting space to a circuit substrate of the switching power supply apparatus is reduced thereby. Further, in the choke-input type rectifying and smoothing circuit, the secondary winding of the transformer for power transmission and the choke coil can be wired as one coil device in the manufacturing stage.
Alternatively, in the switching power supply apparatus including a transformer for power transmission having a primary coil and a secondary coil, a main switch to control the current flowing from an input power supply into the primary coil, a rectifying and smoothing circuit which includes a choke coil and rectifies and smoothes the output of the secondary coil of the transformer for power transmission and a current transformer to detect the current flowing in the main switch and to output it to the above-described control circuit, the transformer for power transmission and the current transformer can comprise the coil device described above.
In this case, two transformers which are formed into one core and constitute a coil device generate no voltage due to the mutual induction between the windings of the two transformers, and are operated as independently individual parts. Thus, a transformer to be used for both power and the control system can have a common core.
Further, in the switching power supply apparatus including a transformer for power transmission having a primary coil and a secondary coil, a main switch to control the current flowing from an input power supply into a primary coil, a rectifying and smoothing circuit having a rectifier which includes a choke coil and rectifies and smoothes the output of the secondary coil of the transformer for power transmission, a control circuit to control the action of the main switch, a drive transformer to transmit the drive signal from a control circuit and drive a high-side switch, and a synchronous commutator the transformer for power transmission and the drive transformer can comprise the coil device described above.
In this case, two transformers which are formed in one core and comprising a coil device generate no voltage due to the mutual induction between the windings of the two transformers, and are operated as independently individual parts. Thus, a transformer to be used both for power and the control system can have a common core.
According to the present invention, since two independent inductors (or coils), a transformer and an inductor, or two independent transformers can be formed in a single coil device using a single core, space can be saved, and the cost can be reduced compared with a case where a plurality of cores for individual inductors are used.
When the number of the total legs of the core is three, cores of the EE-shape, EI-shape, pot-shape or the like which have been extensively used can be used as they are, and the cost for new design of the core can be reduced.
Also, when the coil device wound by a conventional winding is switched to the coil device of the present invention, no bobbin is required, and the cost for the new design can be reduced by using a printed coil printed on a printed circuit board.
Further, by using the coil device of the present invention, it is possible to reduce the space necessary for transformers and coils, minimize the size of the circuit board of a switching power supply apparatus and reduce the mounting cost.
For the purpose of illustrating the invention, there is shown in the drawings several forms which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.